1. Field of Invention
The invention relates to a Sync On Green (SOG) signal detection circuit, and more particularly to a SOG signal detection circuit using a programmable gain amplifier and a low-pass filter (PGA/LPF) to enhance the ability against the noise interference.
2. Related Art
FIG. 1A is a schematic illustration showing a typical video graphics signal. Referring to FIG. 1A, the video graphics signal includes a sync signal A1, a blank zone A2 and a data zone A3. According to the specification, the signal has a level of about 300 mV between the sync signal A1 and the blank zone A2 and a level of about 700 mV between the blank zone A2 and the data zone A3. A video display device typically utilizes a SOG signal detection circuit to detect the sync signal A1 to serve as a control parameter for display.
FIG. 1B shows a conventional SOG signal detection circuit 10. Referring to FIG. 1B, the SOG signal detection circuit 10 includes a clamping circuit 11, a trigger level generator 12 and a comparator 13. The video graphics signal is inputted to the clamping circuit 11 through a capacitor C, and the clamping circuit 11 clamps the video graphics signal within a set voltage range and generates a clamped input signal. The trigger level generator 12 generates a fixed level of trigger voltage. The SOG signal detection circuit 10 further utilizes the comparator 13 to compare the clamped input signal with the trigger voltage and then output a SOG signal.
The SOG signal contains a content of the video graphics signal (i.e., the content of the data zone A3), and the above-mentioned method cannot easily detect the synchronization information. In addition, the power noise and the input noise often interfere with the signal that may be detected. Also, the video graphics signal is coupled to the chip by way of AC coupling, and the clamping voltage thereof is interfered with the noise so that a larger jitter is generated. Meanwhile, the chip typically has the hot plugging function, so the chip also has to hold the normal operation in the short period of time for hot plugging.
In the prior art mentioned hereinabove, the video graphics signal has a level of 300 mV between the sync signal A1 and the blank zone A2, and the smaller noise margin (i.e., the poor ability against the noise interference). If only the trigger level generator 12 is utilized to generate a fixed comparison voltage, the SOG signal cannot be generated effectively.
FIG. 2 shows a SOG signal detection circuit 20 disclosed in U.S. Patent Publication No. 2005/0270421 A1. As shown in FIG. 2, the SOG signal detection circuit 20 includes a clamping circuit 11, a trigger level generator 22 and a comparator 13, and further includes a controller 24. The SOG signal detection circuit 20 can utilize the controller 24 to adjust the comparison voltage generated by the trigger level generator 22 according to the SOG signal. However, the level of the video graphics signal between the sync signal A1 and the blank zone A2 is only about 300 mV, so the noise margin is still smaller and the effect against the noise interference is also poor.